1. Field of the Invention
This invention relates to a toner for developing electrostatic images which is used in printers, copying machines, facsimile machines and so forth employing electrophotography, and an image forming method, a developing apparatus unit and a process cartridge which make use of the toner.
2. Related Background Art
A number of methods are conventionally known for electrophotography. Copies are commonly obtained by forming an electrostatic latent image by various means on a photosensitive member having a photoconductive material, subsequently developing the latent image by the use of a toner to form a visible image (a toner image), transferring the toner image to a transfer medium such as paper if necessary, and thereafter fixing the toner image to the transfer medium by heat and pressure.
In recent years, image forming apparatus employing electrophotography have come to be used not only as conventional copying machines but also as printers and facsimile image output means, and are now in use in ordinary offices and homes.
Copying machines used in offices are always sought to be made more high-speed and stable, and it has become important to ensure stable image density and image quality in their use at a high speed, at the initial stage and even after long-term service. In particular, it is required to deliver stable image quality in an environment of high humidity/high temperature and an environment of low humidity/low temperature, stable image quality immediately after switch-on of the apparatus main body and stable image quality before and after the feeding of toner.
Developing systems for electrophotography are conventionally known to include one-component type developing systems and two-component type developing systems. In particular, one-component type developing systems are being employed in view of the advantage that developing assemblies can be made simple in structure while ensuring a high reliability.
In the one-component type developing systems, however, the toner must be uniformly coated in thin layer on the surface of a roll-shaped toner carrying member such as a developing sleeve by means of a toner layer thickness control member. Hence, a strong rubbing frictional force is applied to the toner to tend to cause deterioration of the toner or deterioration of the surface of the toner carrying member as a result of long-term service, and it is required as a subject to more prevent operating performance from deteriorating. Thus, it is sought to provide a technique for further improving this operating performance.
As charging means in electrophotography, it has been common to use means utilizing corona discharging. Since, however, the use of corona discharging causes ozone in a large quantity, the appratus must have a filter, and there have been the problems that the apparatus must be made large in size and the cost increases.
As techniques for solving such problems, charging methods have been commercialized in which a charging member such as a roller or a blade is brought into contact with the surface of a photosensitive member so as to form a narrow space in the vicinity of the contact portion, and a discharge as can be explained by what is called Paschen's law is formed so that the generation of ozone can be prevented as much as possible. In particular, a roller charging system making use of a charging roller as the charging member is preferably used in view of the stability of charging.
Such contact charging methods and contact transfer methods are disclosed in, for example, Japanese Patent Applications Laid-open No. 63-149669 and No. 2-123385. In these methods, a conductive elastic roller is brought into contact with an electrostatic latent image bearing member, and the electrostatic latent image bearing member is uniformly charged while applying a voltage to the conductive roller, which is then subjected to exposure and developing steps to obtain a toner image. Thereafter, while another conductive roller to which a voltage is applied is pressed against the electrostatic latent image bearing member, a transfer medium is passed between them to transfer to the transfer medium the toner image held on the electrostatic latent image bearing member, followed by the step of fixing to obtain a copied image.
Even in such contact charging methods, however, their fundamental charging mechanisms utilize the phenomenon of discharge from the charging member to the photosensitive member, and hence the voltage necessary for charging as stated above must be at a value beyond the surface potential of the photosensitive member. Moreover, when AC charging is carried out for the purpose of achieving uniform charging, the electric field of AC voltage may cause significant vibration of the charging member and photosensitive member, and noise (hereinafter "AC charging noise"), and the discharge may cause substantial deterioration of the surface of the photosensitive member. This involves another problem of melt adhesion or filming where toner or part of toner components adheres to the surface of the photosensitive member.
In roller transfer systems not employing such corona discharging, a transfer member such as a roller is brought into contact with the photosensitive member via a transfer medium such as paper, and hence filming may also occur because the roller rubs against the toner adhering to the photosensitive member, when the photosensitive member runs idle before and after paper feed, or partial faulty transfer, what is called blank areas caused by poor transfer (hereinafter often "transfer blanks" or "blank images"), may occur because the toner image receives a pressure when the toner image formed on the photosensitive member is transferred to the transfer medium, bringing about great problems on the image quality.
To solve such problems, in Japanese Patent Application Laid-open No. 3-121462, an image forming apparatus is proposed which employs a developer containing a hydrophobic inorganic fine powder treated with silicone oil. However, no improvement has been satisfactorily achieved with respect to thick transfer paper such as post cards and Kent paper having a basis weight of 100 g/m.sup.2 or more and OHP sheets, and it is sought to achieve a much higher image quality.
Since these contact charging member and contact transfer member come in contact with the photosensitive member, lines or uneven tones may also appear in halftone images because the toner remaining after transfer or the toner having escaped from a cleaner adheres to the charging member and the transfer member to inhibit uniform charging and uniform transfer when accumulated in a large quantity.
Toner particles not transferred to the transfer medium and remaining on the photosensitive member are removed in the step of cleaning. In this step of cleaning, blade cleaning, fur brush cleaning or roller cleaning has been conventionally used. These cleaning methods, however, are those in which the toner remaining after transfer is mechanically scraped off or blocked up so that it is collected in a waste toner container. Hence, a cleaning member used therefor which is brought into pressure contact with the surface of the photosensitive member has caused problems. For example, bringing the cleaning member into strong pressure contact causes the surface of the photosensitive member to wear, so that scratches may be produced on the photosensitive member and their marks may appear in images, the toner tends to stick (melt adhere) to the drum surface, and external additives such as silica liberated from the toner may adhere to the drum surface (i.e., filming).